The introduction of exotic arboviruses into new ecological niches has occurred with increasing frequency in recent years, presenting an ongoing challenge to the public health and biodefense communities. RNA viruses, such as West Nile virus, Dengue virus, Rift Valley fever virus and Venezuelan Equine Encephalitis virus, exist within each host as a genetically diverse swarm of competing genotypes that has been termed quasispecies. The quasispecies structure of arbovirus populations allows rapid adaptation of these agents upon inadvertent or deliberate introduction to a new ecological niche. Quantitative measures of the quasispecies nature of zoonotic arbovirus populations in nature do not exist. Further, the impact of transmission by an arthropod on the viral quasispecies has not been adequately assessed. The proposed research will combine field- and laboratory-derived observations to examine the population-level mechanisms of genetic change that may occur when a new arbovirus is introduced into a halve ecosystem. To determine whether the quasispecies structure of arboviruses may promote viral adaptation and disease emergence, proposed research will (a) characterize the quasispecies structure of enzootically transmitted West Nile virus (WNV) and (b) determine whether WNV genetic diversity is generated or constrained in mosquitoes and birds.